1. Field of the Invention
The present invention relates to fibrous media containing millimicron sized particulates, especially media containing fumed silica or fumed alumina.
2. Brief Description of the Prior Art
The technique of flocculating negatively charged filter aid particulates or adsorbents and fibers, by means of positively charged polymers is a common practice in the production of filtration media.
For example, Malcolm, U.S. Pat. No. 3,647,684 describes a felted fibrous matrix containing silicic acid (hydrated silica) interdispersed therein, useful for thin layer chromatography wherein the silicic acid has particle sizes within 1 to 10 microns, and which contains a cationic material in concentrations no higher than 1.5%. Ostreicher, U.S. Pat. Nos. 4,007,113 and 4,007,114 describes a matrix of self-bonding fibers containing interdispersed therein a particulate filter material, the surface of which is modified with an organic colloid. Additional patents relating to fibrous media containing interdispersed particulate material are: Pall et al, U.S. Pat. No. 3,573,158, Leifield, U.S. Pat. No. 3,455,818 and Pall et al, U.S. Pat. No. 3,238,056.
The application of positive charge flocculation techniques for the formation of specialized and improved media has also been described in commonly assigned copending U.S. patent applications Ser. No. 164,797 filed May 30, 1980, Ser. No. 147,975 filed May 8, 198O, now U.S. Pat. No. 4,305,782, and Ser. No. 123,467 filed Feb. 21, 1980, now U.S. Pat. No. 4,309,247, which are herein incorporated by reference.
The porous filter media described in these applications are comprised of fiber-particulate and fiber-fiber mixtures, with cationic charge modifiers serving as charge modifiers, wet strength providers and also as flocculating or dispersion agents in the forming slurry system.
The mixtures are formed dynamically into sheets by vacuum felting from an aqueous slurry, and subsequently drying the finished sheets as the final product. The rate of production of such porous filter media is governed by the porosity of the sheets to be produced. A highly open filter media comprising particle sizes of 50 microns or larger requires only a few seconds to be felted, whereas a tight media utilizing particle sizes of 1-2 microns or smaller would require more than 5 minutes to be felted. Sometimes, media containing the finest grades of particulate additives cannot be felted at all due to improper flocculation. Therefore, it is impossible to form a fibrous media with adsorbent particles less than 1 micron by current techniques. Moreover, the retention of such small size particles in the matrix structure is a serious problem. Most of them are lost to the water drainage during felting.
The limitations and drawbacks involved in current fibrous media manufacturing processes can be understood from the following method of formation. The application of vacuum for the formation of fibrous media is predominantly a hydromechanical process. A slurry containing all the components drains through a 100 mesh wire screen perpendicular to the plane of the screen, and drags all the components with its movement during felting. Large fibers which have the largest surface area in contact with water of any of the components in the slurry, receive the strongest viscous drag force and settle ahead of others to form a bottom fibrous network. This process provides a self-adjusting mechanism for uniform distribution of particles and fibers in the fibrous structure, based upon the fact that the drainage flow always seeks the path of least resistance. While the large fibers are preferentially retained during the initial deposition, smaller fiber fragments tend to migrate through the fiber mat of long fibers and become lodged in the interfiber holes, to provide a coherent mat structure for adsorbent particles to settle in. Continuous application of vacuum after sheet forming, induces mat compaction. The compacting force exerted by the high vacuum, further squeezes water molecules out of the wet pad, and forces adsorbent particles closer together to form a pad with definite porosity.
The above described method, however, is applicable only to particles larger than about 1 micron. In the case of particle sizes less than 1 micron, especially millimicron sized ones, such particles either fail to be retained by the fiber matrix, or fail to be further felted after forming a thin layer of compact particulate.
In particular, the formation of fibrous mats with a high load of millimicron sized particulate (e.g., higher than about 30% by weight) and with high porosity is near to impossible using the felting techniques of the prior art. Among the millimicron sized particles of great interest for commercial chemical and biochemical applications is fumed silica.
The removal of lipids through adsorption on silica is a common practice in chemistry and biochemistry. See e.g. Stephan, U.S. Pat. No. 3,686,395 and commonly asigned copending U.S. patent application Ser. No. 238,686 filed Feb. 27, 1981 to Carpenter and Cone entitled TISSUE CULTURE MEDIA. The efficiency of lipid removal by silica differs with the process by which the silica is made. Silicas precipitated from the vapor phase are better lipid removal agents than those precipitated from sodium silicate solutions. Typical commercial products of silicas made from vapor phase are Cab-O-Sil.RTM., Aerosil.RTM. (Degussa) or Sipernet 22S.RTM.. These products exist in minute particles having average diameters from 7 to 18 millimicrons (or nano-meters). They are produced by the hydrolysis of silicon tetrachloride vapor in a flame of hydrogen and oxygen. At a flame temperature of 1270.degree. K. the vapor pressure of SiO.sub.2 is only 10.sup.-8 Torr, so that there is a very high supersaturation, resulting in large numbers of small nuclei forming silica spheres with diameters ranging from 7 to 18 millimicrons on the average. These molten spheres, termed primary particles, collide and fuse with one another to form branched, three dimensional, chainlike aggregates. During the hydration of fumed silica, hydroxyl groups become attached to some of the silicon atoms on the particle surfaces. This makes the fumed silica surface hydrophilic, and capable of hydrogen bonding with other molecules.
Fumed silica is fluffy and low in density (approximately 2 lbs. per cu. ft.). Even a small amount of fumed silica packed in a column will create extremely high pressures upon contact with buffer solutions, due to the formation of a three dimensional network among the particles, with water molecules functioning as bridges. When prior art methods of dispersal of particulates in fibrous media are applied to fumed silica, the previously mentioned problems of fabrication, retention and porosity are observed.
Fumed silica - albeit not in immobilized form - has been used for the removal of hepatitis B surface antigen from fluids e.g. Stephan U.S. Pat. No. 3,686,395. Other methods of removing viruses from fluids are taught in Porath et al U.S. Pat. No. 3,925,152, Andersson et al U.S. Pat. No. 4,168,300, Wallis et al U.S. Pat. No. 3,770,625, Vnek et al U.S. Pat. No. 3,951,937 and Bick et al U.S. Pat. No. 4,057,628. None of these references uses or suggests immobilized inorganic adsorbants.
At this point it is worth mentioning commonly assigned U.S. Pat. No. 4,228,462 to Hou and Ostreicher for METHOD FOR REMOVING CATIONIC CONTAMINANTS FROM BEVERAGES, which describes a method for preparing a filter sheet having anionic electrokinetic capture potential, which cOmprises cellulose pulp, particulate filter aids, an inorganic cationic surface charge modifier and an inorganic anionic charge modifier, wherein the charge modifiers are cationic and anionic colloidal (inorganic) silicas respectively. Among the particulate aids are mentioned diatomaceous earth, perlite, talc, silica gel, etc., having a high surface area, and being preferably siliceous materials such as the finer grades of diatomaceous earth-perlite. The reason for utilizing the anionic charge modifiers, however, is so as to provide high electrokinetic capture potential for positively charged fluid contaminants. Further, the reason for using inorganic (rather than organic) charge modifiers is so as to prevent the possibility of extracting organic elements into the filtrate. No solution is offered for the production of microparticulate-containing fibrous media.
A need therefore exists for a method of homogeneously immobilizing particulate materials having millimicron sized average diameters, especially fumed silica, into fibrous matrices. A need also exists for the products produced by such method.